Jonathan Franzen's eventful UK visit for his rapturously received new novel, Freedom, took a fresh twist last night, at his launch party at the Serpentine Gallery in London's Hyde Park. An assailant snatched the novelist's glasses from his face and ran off – leaving behind a ransom note asking for $100,000 for their safe return.

Happily, however, the glasses were retrieved after police and a HarperCollins staff member gave vigorous chase, the publisher's communications manager Susanna Frayn said this morning. "Our deputy international sales director Damon Greeney was an absolute hero, and between him and the police they managed to find the glasses," she said. One man was arrested, but Franzen is not pressing charges, Frayn added, dismissing the event as "a harmless prank".

Astonished partygoers recorded yesterday's episode on Twitter under the hashtag #franzen, which rapidly evolved into #glassesgate. "Helicopter above Kensington Gardens, trying to find #Franzen glasses. Apparently miscreants jumped into Serpentine to escape," tweeted the Bookseller's news editor Graeme Neill, adding later, "I can't think of anything over the past few years that was such a mix of shock, disbelief and hilarity." The Guardian's Merope Mills, who was at the event, tweeted that the publisher's speeches were "embarrassingly self-flagellating. 'Jonathan - we're SO SO sorry'." A (fictional) photograph of the missing glasses swiftly appeared for auction on eBay, but was later taken down.

Franzen has attracted a wave of intense press and public interest ever since he offended Oprah Winfrey by expressing reservations about the selection of his novel The Corrections for her famous TV book club in 2001. He has already encountered one major setback with his British publication, after an uncorrected version of the text, accidentally sent to the printers, meant publisher HarperCollins was forced last week to pulp the book's initial print run.